Actually you don't.

I just installed a non-Rockford stock X stereo into my Fosgate equipped X. The original radio was gone, as were all the speakers.

I just had to turn on the amp with an ignition switched power source (tapped off the ignition switched power for the radio itself ) and then installed the radio like normal. The speakers work fine, the radio plays normally and all is good with the universe. The steering wheel controls don't work with the stocker unit (because it's not an RF unit, no plug in for it) but I like the big volume control on the radio, I can adjust it with gloves on.

Now I just have to get my Sirius put into the PhoeniX and go ice racing again. Oh the joy of life.